This invention relates to the purification of acid halides.
In the production of an acid halide, the corresponding aldehyde is generally present as a by-product. For certain uses of the acid halide the aldehyde impurity must be reduced to extremely low levels. This is particularly true in the case of aromatic acid halides when used to prepare aromatic polyketones. Even small amounts of aldehyde present in the monomer can lead to melt instability of the polymer. For example p-phenoxybenzoyl halides, in particular the chloride, are useful in the production of poly(aryl ether ketones) comprising the repeat unit ##STR1## It has been found that in preparing polymers containing this repeat unit from p-phenoxybenzoyl chloride (or other halide), very small amounts of p-phenoxybenzaldehyde or p-phenoxybenzal chloride present as an impurity results in a polymer which tends to be melt unstable, that is, tends to decompose and/or crosslink in the melt, and is therefore not suitable for use for fabrication into articles, coatings, etc. by melt processing techniques.
In the preparation of p-phenoxybenzoic acid, p-phenoxybenzaldehyde is generally present as an undesirable impurity in the product. (Similarly, where other acid halides or dihalides are used, the corresponding aldehydes may be present as undesirable impurities.) If p-phenoxybenzoic acid is converted to p-phenoxybenzoyl chloride, p-phenoxybenzaldehyde and/or its chlorinated derivative, p-phenoxybenzal chloride is present in the product. The content of p-phenoxybenzaldehyde can be reduced by converting it to p-phenoxytoluene which can be removed from p-phenoxybenzoic acid by crystallization. This is done as a separate purification step by reacting the impure p-phenoxybenzoic acid with hydrogen in the presence of a Group VIII metal catalyst or a noble metal catalyst. Such processes are described in U.S. Pat. No. 3,584,039 to D. H. Meyer and U.S. Pat. No. 3,456,001 to G. P. Olsen. Following removal of the p-phenoxytoluene the p-phenoxybenzoic acid is converted to p-phenoxybenzoyl chloride and then distilled. The p-phenoxybenzaldehyde and p-phenoxybenzal chloride cannot be removed in the distillation step as they co-distill with p-phenoxybenzoyl chloride. Thus the separate step of converting it to p-phenoxytoluene has been required by the prior art.
We have now discovered a process by which, for example, p-phenoxybenzaldehyde can be removed from impure p-phenoxybenzoyl chloride during distillation or prior to distillation, preferably in the distillation vessel. The term p-phenoxybenzaldehyde is used herein to refer to the compound per se and its chloro derivative present in the impure p-phenoxybenzoyl chloride.